GB2304008A

GB2304008A - Radio receiver comprising an expander

Info

Publication number: GB2304008A
Application number: GB9515640A
Authority: GB
Inventors: Gadi Shirazi; Tal Mor; Zami Schwartzman
Original assignee: Motorola Israel Ltd
Current assignee: Motorola Solutions Israel Ltd
Priority date: 1995-07-31
Filing date: 1995-07-31
Publication date: 1997-03-05
Also published as: GB9515640D0

Abstract

A radio receiver includes a plurality of receiver sub-circuits 12-18 operably coupled for receiving an input signal (11) and for providing a baseband output signal (17). A switching element (22) is operably coupled to an audio enhancement element i.e an expander (20) for receiving the baseband output signal (17) and for selectively routing the baseband audio signal (17) to the audio enhancement element (20) and for receiving an improved audio baseband output signal therefrom. An audio enunciating element i.e. a speaker (24) is operably coupled to the switching element (22) for receiving the improved audio baseband output signal and enunciating speech. A micro controller 26 monitors a signal strength or signal quality signal 34 and when it falls below a threshold applies a control signal 32 to the switch 22 to apply the audio signal 17 directly to the speaker 24 thus avoiding undesirable effects due to noise spikes in poor quality audio signals.

Description

RADIO RECEIVER AND METHOD OF OPERATION Field of the Invention This invention relates to receiver designs. The invention is applicable to, but not limited to, radio receivers of mobile radio equipment operating in companding communications systems.

Background of the Invention A common method for improving audio quality in a radio communication system, is to use companders. This method involves compressing the dynamic range of an audio signal in a transmitter and subsequently expanding the received signal in the receiver to recover the original audio signal. The compressing and expanding functions are accomplished by using an envelope dependent gain stage. In the compressor the gain is higher for low envelope signals and in the expander the gain is lower for high envelope signals. The compander method improves a receivers signal to noise ratio (SNR), as long as the unprocessed SNR is larger than unity.

In frequency modulated (FM) radio communication systems, even in a tolerable SNR range of 10 to 20 dB, large noise spikes known as FM "clicks" can occur. These spikes may be much larger than the received audio signal and if such spikes are input to an expander in a companding receiver, the clicks are amplified to an intolerable level. Thus, while greatly improving audio quality under good reception conditions, the effective coverage range for a particular received signal level is reduced by such intolerable noise spikes.

Thus, there exists a need in companding radio communications receivers for a receiver design and method of operation that provides improved audio quality in normal SNR conditions, while providing intelligible speech over extended range even under marginal reception.

Summarv of the Invention In a first aspect of the present invention, a radio receiver is provided. The radio receiver has a receiver circuit that includes a plurality of receiver sub-circuits operably coupled for receiving an input signal and providing a baseband output signal. A switching element is operably coupled to an audio enhancement element for receiving the baseband output signal, for selectively routing the baseband output signal to the audio enhancement element and for receiving an improved audio baseband output signal therefrom. An audio enunciating element is operably coupled to the switching element for receiving the improved audio baseband output signal from the switching element and for enunciating speech.

In a preferred embodiment, the radio receiver includes a micro controller operably coupled to at least one receiver sub-circuit for receiving at least one received input signal characteristic, i.e. a received signal strength indication or an audio quality indication, from the least one receiver sub-circuit. The micro controller is operably coupled to the switching element for providing a control signal to the switching element dependent upon the at least one received input signal characteristic for selectively switching the received baseband output signal to the audio enhancement element.

In this manner, the micro controller selects when an input signal would benefit from audio enhancement and advantageously controls the operation of the switching element to provide this feature.

In a second aspect of the present invention a radio receiver is provided. The radio receiver has a receiver circuit comprising a plurality of receiver sub-circuits operably coupled for receiving an input signal and providing a baseband output signal. An audio enhancement element is provided for receiving the baseband output signal and for selectively providing an improved audio baseband output signal. A micro controller is operably coupled to at least one receiver sub-circuit for receiving at least one received input signal characteristic, e.g. a received signal strength indication or an audio quality indication, from the at least one receiver sub-circuit and for providing an audio control signal for selectively disabling the operation of the audio enhancement element dependent upon the at least one received input signal characteristic.An audio enunciating element is operably coupled to the audio enhancement element for receiving audio baseband output signals and enunciating speech therefrom.

In this manner, the audio enhancement element is selectively disabled by the microcontroller when audio enhancement of the received signal is not beneficial.

Preferably the radio receiver, according to the first and second aspects of the invention, operates in a companding communications system and the audio enhancement element is a compander used for selectively expanding the audio dynamic range of the received signal.

Advantageously, the audio quality of the received signal is enhanced in conditions of low signal to noise levels.

In a third aspect of the present invention, a method of operating a radio receiver is provided. The radio receiver includes a plurality of receiver sub-circuits, an audio enhancement circuit and an audio enunciating element. The method comprises the steps of receiving an input signal at a first receiver sub-circuit wherein the first receiver subcircuit is operably coupled to at least a second receiver sub-circuit. A baseband audio output signal is provided from the second receiver subcircuit to the audio enhancement circuit and the baseband audio output signal is selectively disabled from the audio enhancement element. The baseband audio output signal is transmitted to the audio enunciating element.

In a preferred embodiment of the third aspect of the present invention, the radio receiver includes a micro controller operably coupled to at least one receiver sub-circuit and to the audio enhancement circuit.

The method further comprises the steps of receiving at least one input signal characteristic at the micro controller from the at least one receiver sub-circuit and transmitting a control signal from the microcontroller to the audio enhancement circuit for selectively disabling the baseband audio output signal from the audio enhancement element dependent upon the at least one received input signal characteristic.

Brief Description of the Drawings FIG. 1 is a block diagram of a radio receiver architecture employing an expander and a switching element according to a preferred embodiment of the invention.

FIG. 2 is a flow chart showing the switching method of an expander according to a preferred embodiment of the invention.

Detalled Descnption of the Drawings Referring first to FIG. 1, a block diagram of a radio receiver having an audio enhancement element and a switching element 22 according to a preferred embodiment of a first aspect of the invention is shown.

The radio receiver comprises a receiver element 10 , a switching element 22, an audio enhancement element, e.g. an expander, 20, an audio amplifier 23, an audio enunciating element, e.g. a loudspeaker, 24 and a micro controller (P) 26. The receiver element 10 comprises a number of receiver sub-circuits. The receiver sub-circuits include a front end filter (FE) 12, a local oscillator (LO) 13, a mixer 14, a band pass filter (BP) 16 and a back end filter with demodulator 18. The pP 26 comprises an analog to digital converter (ADC) 28, which is operably coupled via a control line 32 to the switching element 22, and coupled via a control line 30 to the expander 20.A receive signal 11, a baseband audio signal 17 and a received input signal characteristic, e.g. a receiver signal strength indicator (RSSI) 34 or an audio quality indicator, are also shown.

In operation, the receive signal 11 is fed into the front end filter 12 and the filtered receive signal is input into the mixer 14 where it is mixed with a local oscillator 13 frequency to provide a low frequency signal. The low frequency (baseband audio) signal is fed into the band pass filter 16 and then into the back end filter with a demodulator 18 to provide a baseband output signal 17. The baseband output signal 17 is input to a switching element 22 which is operably coupled to an expander 20 for receiving the baseband output signal 17 and for selectively routing the baseband output signal 17 to the expander 20 and for receiving an improved audio baseband output signal therefrom. A loudspeaker 24 is operably coupled to the switching element 22 for receiving the improved audio baseband output signal from the switching element 22 and enunciating speech.

In a preferred embodiment, the receiver element 10 provides an audio signal 17 to the switching element 22 and at least one received input signal characteristic, e.g. RSSI signal 34, to the ADC 28. The ADC 28 measures the RSSI signal 34 and when the level of the RSSI signal 34 is such that there is no benefit in audio enhancement, i.e. marginal reception condition, the ptP 26 provides a control signal 32 to the switching element 22, dependent upon the RSSI signal 34, for selectively switching the baseband audio signal through the switching element 22 to the audio amplifier 23 and subsequently to the loudspeaker 24. The gP controls the operation of the switching element 22 via the control line 32.In higher levels of RSSI signal 34, at which the audio enhancement is beneficial the baseband audio signal 17 is fed into the expander 20, where the signal level is expanded and the audio quality enhanced, and an enhanced baseband audio signal is input into the switching element 22, and subsequently audio amplifier 23 and loudspeaker 24.

Preferably, the radio receiver is a companding radio receiver for operation in a companding communications system.

It is within the contemplation of the invention that the switching element 22 and expander 20 may be replaced by an expander circuit, wherein the expander circuit is selectively disabled by the I1P 26 via a control line 30 when the received signal level is unacceptable: Referring now to FIG. 2, a flow chart is shown, illustrating -a method for switching an audio signal through an expander for certain received signal conditions, in accordance with the preferred embodiment of the invention.

The radio receiver has a plurality of receiver sub-circuits, an audio enhancement circuit and an audio enunciating element. The method includes the steps of receiving an input signal at a first receiver sub-circuit wherein the first receiver sub-circuit is operably coupled to at least a second receiver sub-circuit. A baseband audio output signal is provided from the second receiver sub-circuit to the audio enhancement circuit.

The baseband audio output signal is selectively disabled from the audio enhancement element and the baseband audio output signal is transmitted to the audio enunciating element.

Preferably, the radio receiver further includes a micro controller operably coupled to at least one receiver sub-circuit and the audio enhancement circuit. Preferably, the method further includes the steps of receiving at least one input signal characteristic at the micro controller from the at least one receiver sub-circuit and transmitting a control signal from the microcontroller to the audio enhancement circuit for selectively disabling the baseband audio output signal from the audio enhancement circuit dependent upon the at least one input signal characteristic.

In FIG. 2, the receiver element 10 has a plurality of receiver subcircuits wherein the receiver element 10 receives and demodulates a receive signal 11 as in step 100 to provide a baseband audio signal 17 and a received signal characteristic e.g. a RSSI signal 34. The ADC 28 measures the RSSI signal 34, as in step 104. The llP 26 compares the RSSI signal 34 with predetermined values as shown in step 106, and when the measured RSSI signal 34 is below the predetermined values of typically corresponding to 10 dB SNR the RP 26 selectively disables the baseband audio signal 17 from the audio enhancement element, e.g. an expander 20, as shown in step 108.The llP 26 controls the switching element 22 to route the baseband audio signal 17 to the audio amplifier 23 and an audio enunciating element, e.g. a loudspeaker 24.

When the RSSI signal 34 is not above the predetermined values, the cup 26 routes the baseband audio signal 17 to the expander 20, as in step 110, where it is expanded, enhancing the audio quality, and subsequently input to the audio amplifier 23 and to loudspeaker 24, as shown in step 108.

Receivers currently exist in cellular communication and trunked mobile radio applications that provide received signal characteristics such as RSSI or, for example, an audio quality indication used for the purpose of noise activated squelch. Advantageously, such received signal characteristics are monitored signals, and used to disable the expander if the received signal level or audio quality drops below a pre determined level. The disabling of the expander in low received signal levels enables the user to benefit from the improved SNR under good signal conditions, whilst still receiving intelligible conversation under marginal reception condition.

Thus, a receiver design and a method of operation for a companding radio communications system are provided that offer improved audio quality and extended operating range.

Claims

1. A radio receiver having a receiver circuit comprising: a plurality of receiver sub-circuits operably coupled for receiving an input signal and providing a baseband output signal; a switching element operably coupled to an audio enhancement element for receiving the baseband output signal and for selectively routing the baseband output signal to the audio enhancement element and for receiving an improved audio baseband output signal therefrom; and an audio enunciating element operably coupled to the switching element for receiving the improved audio baseband output signal from the switching element and enunciating speech.

2. A radio receiver according to claim 1, further comprising a micro controller operably coupled to at least one receiver sub-circuit for receiving at least one received input signal characteristic from the at least one receiver sub-circuit and operably coupled to the switching element for providing a control signal to the switching element dependent upon the at least one received input signal characteristic for selectively switching the baseband output signal to the audio enhancement element.

3. A radio receiver having a receiver circuit comprising: a plurality of receiver sub-circuits operably coupled for receiving an input signal and providing a baseband output signal; an audio enhancement element for receiving the baseband output signal and for selectively providing an improved audio baseband output signal; a micro controller operably coupled to at least one receiver subcircuit for receiving at least one received input signal characteristic from the at least one receiver sub-circuit and for providing an audio control signal for selectively disabling the audio enhancement element dependent upon the at least one received input signal characteristic; and an audio enunciating element operably coupled to the audio enhancement element for receiving audio baseband output signals and for enunciating speech.

4. A radio receiver according to claim 2 or 3, wherein the at least one received input signal characteristic is a received signal strength indication.

5. A radio receiver according to claim 2 or 3, wherein the at least one received input signal characteristic is an audio quality indication.

6 A radio receiver according to any of the preceding claims, wherein the radio receiver operates in a companding communications system and wherein the audio enhancement element is a compander used for selectively expanding audio dynamic range of the baseband output signal to enhance the audio quality of the baseband output signal in conditions of signal to noise such that would benefit from the audio enhancement.

7. A method of operating a radio receiver having a plurality of receiver sub-circuits, an audio enhancement circuit and an audio enunciating element, the method comprising the steps of: receiving an input signal at a first receiver sub-circuit wherein the first receiver sub-circuit is operably coupled to at least a second receiver sub-circuit; providing a baseband audio output signal from the second receiver sub-circuit to the audio enhancement circuit; disabling selectively the baseband audio output signal from the audio enhancement element; and transmitting the baseband audio output signal to the audio enunciating element.

8. A method of operating a radio receiver according to claim 7, wherein the radio receiver further comprises a micro controller operably coupled to at least one receiver sub-circuit and the audio enhancement circuit wherein the method further comprises the steps of: receiving at least one input signal characteristic at the micro controller from the at least one receiver sub-circuit; and transmitting a control signal from the microcontroller to the audio enhancement circuit for selectively disabling the baseband audio output signal from the audio enhancement circuit dependent upon the at least one input signal characteristic.